The present application relates to a microporous film with a laminate structure, and in more detail, relates to a laminated microporous film, which includes a base formed of a non-woven fabric and a surface layer, which includes inorganic particles and where a heating and pressurizing process has been carried out, and which is formed so that the entire thickness is uniform, a battery separator, and a non-aqueous electrolyte battery which uses the laminated microporous film and the battery separator.
In recent years, along with the spread of the use of portable information electronic apparatuses such as mobile phones, video cameras, laptop computers, higher performance, smaller sizes, and lighter weights of these apparatuses have been achieved. A disposable primary battery or a secondary battery which is able to be used repeatedly are used as the power sources of these apparatuses, but for excellent comprehensive balance in terms of higher performance, smaller sizes, lighter weights, and economy efficiency, the demand for secondary batteries, and in particular lithium ion secondary batteries is increasing. In addition, in these apparatuses, there has been further progress in terms of higher performance and lighter weights, and higher energy density is demanded in relation to lithium ion secondary batteries. In addition, due to the high energy density, lithium ion secondary batteries are also used as batteries mounted in electric power tools, electrically-powered bicycles, electric cars, hybrid cars, and the like.
In the case of being used in cars, harsh usage is assumed such as higher battery capacity, larger currents with regard to rapid charging and discharging, and repeatedly charging and discharging over a long period of time. In the past, in the secondary batteries which are mainly held in small apparatuses, a microporous film which is formed from polyolefin resin is adopted as a separator. Among these, safety is ensured by the polyolefin microporous film melting at approximately 130° C. and the porous holes being clogged, that is, by having a so-called shutdown function. Furthermore, in a laminated microporous film with polyethylene and polypropylene, it is possible that the melt-down temperature is increased due to the high melting point of polypropylene and high levels of safety are realized using a single layer film of polyethylene. However, in a case of an electric power tool and use in a car where large currents are used, the heat resistance of the polyethylene resin is insufficient and safety at higher temperatures is demanded and non-woven fabrics formed from materials with high melting points are often examined.
The non-woven fabric is used as a relatively cheap microporous film in a low-quality battery. However, in the manufacturing method, since unevenness in the thickness is large and pin holes are generated in an irregular manner, the adoption in secondary batteries has not begun in earnest. In addition, it may be pointed out that there is a drawback in that the mechanical characteristics of non-woven fabric are weak in the structure where microfibers are entwined.
In regard to the drawbacks of non-woven fabric, it has been disclosed that uniformity is possible in regard to the unevenness in thickness by performing a calendar process with regard to a surface of non-woven fabric as shown in Japanese Unexamined Patent Application Publication No. 2004-87228. On the other hand, there is no other measure for countering the generation of irregular pin holes which is solved by a single layer of non-woven fabric other than minimizing the probability of generation of the pin holes by increasing the amount of lamination of the microfibers and increasing thickness. It is not a desirable countermeasure as, while the number of holes is small, the existence of holes which may cause a short is not able to be permitted in terms of the safety of the battery and a thick separator acts against the higher performance of the battery.
In regard to this, in a separator which has a laminated structure using a non-woven fabric as a base, a method has been proposed where the pin holes are covered up by a PVdF resin layer which is included in inorganic particles being formed on the surface of the non-woven fabric as shown in Japanese Unexamined Patent Application Publication (Translation of PCT Application) No. 2005-536658. In addition, in regard to the insufficient mechanical characteristics, there is improvement by the PVdF being bonded with the microfibers of the non-woven fabric.